TOKYO Japan's health ministry said it would investigate reports that workers at the stricken Fukushima nuclear power plant were urged by a subcontractor to place lead around radiation detection devices in order to stay under a safety threshold for exposure.

The Asahi Shimbun newspaper reported on Saturday that an executive from Build-Up, a subcontractor to plant owner Tokyo Electric Power, told workers to cover the devices called dosimeters when working in high-radiation areas.

Dosimeters can be worn as badges or carried as devices around the size of a smart phone to detect radiation.

Nine workers wore the lead plates around the devices once after the executive's plea, Public broadcaster NHK said, citing the subcontractor's president.

Japanese law has set an annual radiation exposure safety threshold of 50 millisieverts for nuclear plant workers during normal operations.

But a massive earthquake and tsunami that crippled the Fukushima plant in March 2011 led to a breach of containment structures that released radiation, keeping large areas around the plant off limits more than a year later.

A Tokyo Electric Power spokesman told Reuters on Saturday the company was aware from a separate contractor that Build-Up made the lead shields, but that they were never used at the nuclear plant.

Build-Up could not be reached for comment.

(Reporting by Osamu Tsukimori; Editing by Ed Lane)

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